1. Field of the Invention
The present invention relates to a vital rate decoder for actuating any one of a plurality of output relays in correspondence with the frequency or rate code of an input signal applied to the decoder, which input signal may be generated, for example, by my "Vital Electronic Code Generator" as disclosed in my co-pending U.S. application Ser. No. 002,765 filed Jan. 11, 1979, the disclosure of which is hereby incorporated by reference.
Also, the present invention more generally relates to the technology disclosed in my prior U.S. Pat. Nos. 4,090,173 and 3,995,173, and my prior U.S. patent application Ser. No. 873,574 filed Jan. 30, 1978, now U.S. Pat. No. 4,181,849, and Ser. No. 002,765 filed Jan. 11, 1979, the disclosure of each of which is incorporated by reference herein.
2. Description of the Prior Art
In a number of industrial applications, such as railroad technology, code generators are commonly used to transfer information. In the railway signaling and control field, for example, vehicle speed, application of motor and braking power, indicator lights aboard the vehicle, and other functions may be controlled automatically or manually in response to coded information transmitted from wayside stations via the rails. This coded information normally takes the form of low frequency pulses having frequencies corresponding to the particular control functions which are applied by a code generator to the vehicle rails in the form of variable low frequency pulse rates, which are then detected by a decoder at another station programmed to detect the frequency, and operate relays in conjunction with the frequency of the detected incoming pulse train.
Since human lives often depend upon safe operation of the vehicle and which in turn depends upon reliable, accurate detection of the pulse rate signals applied to the vehicle rails, railway control systems are typically required to exhibit fail-safe or "vital" qualities. To that end, modern rail transit systems employ cycle checking and diversity safety design techniques to protect against unsafe conditions. Cycle checking involves a continuous testing of a device, circuit or computer instruction to ensure that it is completely functional. Diversity, on the other hand, involves the use of two or more independent channels to produce a permissive output, in which the channels are selected so that a single disruptive event cannot cause identical failures in all of the channels, and all of the channels must agree before permissive output is accepted. These safety design techniques are directed to the promotion of a fail-safe or "vital" operation, in which any failures which occur tend to result in a condition which is no more dangerous (or conversely at least as safe) as if an equipment failure has not occurred.
In the past, the task of decoding the various pulse rates, typically on the order of 75, 120 or 180 pulses per minute, applied to the rails has been performed by passive LC filter circuits, each tuned to a particular pulse repetition rate, and each formed of very large inductor and capacitor components. Thus, at least one tuned circuit for each pulse rate has been required. However, because of the low frequencies involved, massive inductors and capacitors, which are expensive, extremely heavy, bulky, and take up considerable space, have been employed. Thus, although the previously used passive circuits are generally reliable in that the tuned frequency of such a passive circuit is not easily subject to change, nevertheless the obvious disadvantages associated therewith have resulted in efforts to produce lighter, smaller, less expensive, and equally reliable alternatives. One alternative employed involves modern active filtering techniques, typically using an operational amplifier and associated resistors and capacitors arranged in a feedback circuit resulting in a filter tuned to the particular frequency. While active filters of this type are considerably smaller and less expensive, they achieve these improvements at the expense of reliability, and more particularly at the expense of the assurance of failure-free performance, in view of the higher likelihood of a failure in an active filter circuit and in view of the difficulty in implementing cycle checking and diversity features in these types of circuits.